Method and apparatus for producing an absorbent structure

ABSTRACT

A method for producing an absorbent structure for absorbent sanitary articles, comprising the steps of: advancing a first non-woven layer at a first speed, with a first surface of said first non-woven layer facing a suction chamber, distributing superabsorbent granular material on a second surface of said first non-woven layer opposite to said first surface, volumizing said first non-woven layer by means of a toothed portion that penetrates into said first non-woven layer through said second surface and movable with respect to said first non-woven layer with a speed difference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Italian patent application number 102015000058406, filed Oct. 6, 2015, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to absorbent sanitary articles and regards a method and an apparatus for producing an absorbent structure for absorbent sanitary articles.

Description of Prior Art

The designation “absorbent sanitary articles” generally indicates articles such as, for example, diapers and pull-ups for infants and children, incontinence pads, sanitary napkins for women, and similar articles, intended to absorb body fluids.

These sanitary articles typically have a layered structure comprising an outer sheet or backsheet impervious to liquids, an inner sheet or topsheet permeable to liquids and intended to be placed in contact with the user's skin, and a central absorbent insert or core that has the function of capturing and storing body fluids.

In the first industrial diapers that appeared on the market in the late 1950s, the absorbent core was mainly composed of cellulose fluff with different shapes and thicknesses. Subsequently, in the second half of the 1980s the tendency developed to add superabsorbent granular materials—with a high degree of fluid retention- to the cellulose fluff.

These superabsorbent materials are known by various names such as, for example, SAP (acronym for Super Absorbent Polymer) or AGM (acronym for Absorbent Gelling Material). In most cases, hydro-gelling materials are used, which are capable of absorbing and capturing the liquid in a mostly stable manner.

By way of reference, one gram of cellulose fluff is capable of absorbing 8-10 grams of 0.9% saline solution, but it only retains a very small fraction of it, typically 2-3 grams. One gram of superabsorbent material is able to absorb about 50 grams of saline solution and to retain about 30 grams of it after centrifugation (EDANA method 441.1-99) or about 22 grams under a load of 0.7 psi (EDANA method 442.1-99).

In view of these characteristics, the idea of producing an absorbent core for sanitary articles composed primarily, if not exclusively, of superabsorbent material, certainly appears attractive.

Absorbent structures composed almost exclusively of superabsorbent material are known, and are described in numerous patent documents such as U.S. Pat. Nos. 4,600,458, 4,658,914, 4,681,577, 4,685,914, 5,643,238, 593,650 and WO-A-2005/004939.

One of the major difficulties in producing absorbent structures without fluff is that of combining together the integrity and the absorption capacity of the liquid of the structure. An absorbent core without fluff should have very high values of integrity and of absorption capacity. In reality, what happens is that if priority is given to one of the two characteristics, the other consequently results as being penalized, or rather a high absorption capacity is achieved at the price of an insufficient integrity of the structure and vice versa.

Methods and apparatus have already been proposed that have the object of penetrating granular material within a layer of highly voluminous fibers, in order to produce an absorbent structure without fluff. For example, WO2013/153235 describes a method for producing an absorbent structure comprising at least one fibrous non-woven layer having an empty volume configured to be penetrated by superabsorbent particles. The superabsorbent particles are dispersed into the fibrous layer by means of vacuum and vibrations.

The technology which provides the penetration of superabsorbent particles into a fibrous layer by means of vacuum and vibrations is not sufficiently effective because, in practice, it does not allow a deep penetration of the superabsorbent material granules into the fibrous layer and, consequently, does not allow the distribution of high amounts of superabsorbent material into the fibrous layer.

EP-A-1526214 describes an electrostatic method for the penetration of superabsorbent powder within a fibrous layer. The powder and the fibrous layer are placed between electrodes, which are connected to the poles of an alternating current generator. The electrodes form an electrostatic field with a voltage gradient in the order of 0.10-20 kV/mm.

In practice, the electrostatic penetration technology provides better results than the penetration technology with vacuum and vibrations. However, the electrostatic technology is affected by various drawbacks, including the high length of the electrostatic tunnel (in the order of 10 meters), the high cost and safety issues related to the risk of subjecting inflammable material to a high electrical voltage (up to 20 KV).

EP-A-0540041 describes a method for forming a super-absorbent composite material, which comprises a step of hydraulic needling of a sheet of non-woven fabric to increase its distribution properties of the liquids, and to introduce dry superabsorbent material in intimate contact with at least one surface of the hydraulically-needled fibrous sheet. This document describes that the composite sheet may be mechanically softened by calendering, opening, embossing, differential ironing, etc.

SUMMARY OF THE INVENTION

The present invention aims to provide a method and an apparatus for producing an absorbent structure without fluff, composed almost exclusively of fibrous material and superabsorbent granular material, which is provided with a high degree of integrity in combination with a high capacity for absorbing body fluids, and that overcomes the previously exposed problems of the prior art.

According to the present invention, this object is achieved by a method and by an apparatus having the characteristics forming the subject of claims 1 and 15.

The claims form an integral part of the disclosure provided here in relation to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to the attached drawings, given purely by way of non-limiting example, wherein:

FIG. 1 is a schematic cross-section of an absorbent structure without fluff;

FIG. 2 is a schematic view illustrating an apparatus for producing the absorbent structure of FIG. 1;

FIG. 3 is a schematic view illustrating a second embodiment of an apparatus for producing the absorbent structure of FIG. 1;

FIG. 4 is a schematic view illustrating a second embodiment of an absorbent structure without fluff;

FIG. 5 is a schematic view illustrating an apparatus for producing the absorbent structure of FIG. 4; and

FIG. 6 is a schematic view illustrating an apparatus for producing the absorbent structure with the in-line formation of a non-woven sheet.

DETAILED DESCRIPTION

With reference to FIG. 1, numeral 10 indicates an absorbent structure without fluff intended to form absorbent cores for disposable absorbent sanitary articles.

The absorbent structure 10 comprises a first non-woven layer 12, a superabsorbent granular material 14, and a second non-woven layer 16. The superabsorbent granular material 14 is dispersed between the fibers of the first non-woven layer 12. The first and the second non-woven layers 12, 16 are permeable to bodily fluids that the absorbent structure 10 is intended to absorb.

The second non-woven layer 16 is optional, so that the absorbent structure 10 could be formed by the first non-woven layer 12 and the superabsorbent granular material 14 only.

The first non-woven layer 12 has a first main surface 18 and a second main surface 20. The second non-woven layer 16 is permanently attached to the second main surface 20 of the first non-woven layer 12 by gluing or welding. The second non-woven layer 16 can have a thickness that is substantially less than the thickness of the first non-woven layer 12. In the second non-woven layer 16, superabsorbent granular material is not typically present.

The absorbent structure 10 is produced in the form of a continuous tape elongated in a longitudinal direction. The continuous absorbent structure 10 is intended to be cut transversely into a plurality of sections, each of which forms an absorbent core of a disposable absorbent sanitary article. Each absorbent core obtained by the absorbent structure 10 is intended to be enclosed between a permeable topsheet and an impermeable backsheet of an absorbent sanitary article. Preferably, the second non-woven layer 16 is intended to face towards the topsheet and the first surface 18 of the first non-woven layer 12 is intended to face towards the backsheet. However, the fact that the second non-woven layer 16 is intended to face towards the topsheet and the first non-woven layer 12 is intended to face towards the backsheet does not detract genericity from the description and does not have specific relevance regarding the production modes of the absorbent structure 10. Therefore, absorbent structures that are incorporated in an absorbent sanitary article with the first layer 12 facing towards the user, during use, and with the second layer 16 facing outwards are also possible.

In the case in which the second non-woven layer 16 faces towards the topsheet, it can perform the function of a liquid acquisition and distribution layer (ADL), which absorbent cores are typically provided with. Typically, body fluids are fed into a concentrated area of the absorbent core. The ADL spreads the liquids over a larger surface area of the absorbent core and promotes a more uniform absorption of liquids by the superabsorbent material 14 dispersed in the first non-woven layer 12.

The first non-woven layer 12 is a highly voluminous fibrous material that can have average specific volume values between a minimum of 25 cm³/g and a maximum of 120 cm³/g, preferably between 30 cm³/g and 100 cm³/g, more preferably between 40 cm³/g and 80 cm³/g and with a thickness that can be between 2 and 12 mm, preferably between 2 and 8 mm, more preferably between 3 and 6 mm.

Suitable materials for the first non-woven layer 12 can be carded non-woven fabrics such as through-air bonded fabrics with bicomponent fibers. An example of a suitable material is Airten TL Phil that has a specific volume of 44.5 cm³/g and a thickness of 4 mm produced by PGI of Charlotte N.C., USA. A further example of a suitable material to be used as the first non-woven layer 12 can be a through-air bonded hydrophilic non-woven fabric of 80 g/m² with a thickness of 3.2 mm that has an average specific volume of about 40 cm³/g, identified with the code Vortex 80, produced and marketed by Texsus Spa, via Livornese di Sopra, 82-51013 Chiesina Uzzanese, Pistoia, Italy.

The second non-woven layer 16 can have a grammage between 8 and 20 g/m² and a thickness of between 0.1 and 0.4 mm. A suitable material to be used for the second non-woven layer 16 is the SMS-type non-woven hydrophilic fabric of 10 g/m² with a thickness of 0.1 mm identified with the code IC3EW-100 010 DB W, produced and marketed by Fiberweb Nonwovens S.r.l Trezzano Rosa, Milano, Italy.

For these types of materials, the thickness values can be determined directly with the ISO 9073-2 method or with one of the equivalent methods EDANA WSP 120.6.R4 or ERT 30.05.99. The mass per unit area can be determined using one of the following methods that are equivalent to each other: ISO 9073-1, EDANA WSP 130.1.R4 or ERT 40.3.90. The specific volume can be derived by relating the thickness and the mass per unit area acquired using one of the aforementioned methods.

One superabsorbent granular material 14 suitable for use in the absorbent structure 10 is FAVOR Z 3403, produced and marketed by Evonik Industries AG of Krefeld, Germany, which has an apparent density (bulk density) equal to 700 g/dm³.

With reference to FIG. 2, numeral 22 indicates an apparatus for producing the absorbent structure 10, previously described. The apparatus 22 comprises a transport device 24 configured for advancing the first non-woven layer 12 at a first speed V1. In the example illustrated in FIG. 2, the transport device 24 is a wheel rotating around its own central axis and having an outer surface 26 that moves with a peripheral speed equal to the advancing speed V1 of the first non-woven layer 12. The outer surface 26 of the transport device is perforated and faces a stationary suction chamber 28 located inside the outer surface 26, and which produces a vacuum on the section of the outer surface 26 facing the suction chamber 28.

The first non-woven layer 12 is fed onto the outer surface 26 of the transport device 24. The first non-woven layer 12 is held on the surface 26 by the vacuum produced by the suction chamber 28. The first surface 18 of the first non-woven layer 12 faces the outside surface 26 of the transport device, and the second surface 20 of the first non-woven layer 12 faces outwards. The first non-woven layer 12 is a continuous tape that can be fed from a reel 30 or can be formed in line, as will be described below.

The apparatus 22 comprises a stationary dispensing device 32 that feeds measured quantities of superabsorbent granular material 14 onto the second surface 20 of the first non-woven layer 12. The superabsorbent granular material 14 can be distributed in the first non-woven layer 12 in a uniform manner. However, it is also possible to distribute the superabsorbent granular material 14 in the first non-woven layer 12 with non-uniform dosages in the longitudinal direction and/or in the transverse direction, so that the superabsorbent granular material 14 is distributed with different densities in different areas of the absorbent core in order to modulate the absorption characteristics of the various areas of the absorbent core.

For example, the superabsorbent material may be distributed along longitudinal bands (“channels”) parallel to each other and separated by areas without superabsorbent material.

An alternative configuration for distributing the superabsorbent granular material 14 in the first non-woven layer 12 with non-uniform dosages consists of an intermittent distribution of the superabsorbent material in the longitudinal direction at a predetermined pitch, so as to form transverse areas of the first non-woven layer 12 provided with the superabsorbent granular material 14 and spaced apart by transverse areas of the first non-woven layer 12 without superabsorbent granular material 14. To carry out this non-uniform distribution known means can be used, which replace the dispensing device 32. Specifically, for the application of the superabsorbent granular material 14 in an intermittent manner in the longitudinal direction, the apparatus described in EP-A-2777664 by the same Applicant could be used to dispense the superabsorbent granular material 14.

The apparatus 22 comprises a first volumizing device 34 preferably arranged downstream of the dispensing device 32. In the embodiment illustrated in the figures, the first volumizing device 34 comprises a cylinder 36 having a toothed portion 38 equipped with protruding elements.

The volumizing device 34 can be composed of a cylinder formed of a series of toothed disks placed next to each other along the axis of the cylinder, in a manner similar to the rotor of a defibrator mill for cellulose, on a much smaller scale. In general, the volumizing device 34 can have an essentially cylindrical shape or, alternatively, can comprise a tape having a flat outer surface, and provided on the toothed portion 38 with outwardly-projecting elements, preferably radial, for example of metal or of synthetic material, such as protuberances, nails or teeth.

In general, the toothed portion 38 of the first volumizing device 34 is movable relative to the first non-woven layer 12, so that there is a difference in speed ΔV between the speed V1 of the first non-woven layer 12 and the speed of the toothed portion 38.

In the preferred embodiment illustrated in FIG. 2, the toothed portion 38 of the first volumizing device 34 is preferably movable in a direction concordant to the direction of movement of the first non-woven layer 12 at a speed V2 lower than the speed V1 of the first non-woven layer 12. Alternatively, the toothed portion 38 may also be movable in the opposite direction to the direction of movement of the first non-woven layer 12.

The toothed portion 38 is located at a distance from the outer surface 26 of the transport device 24 that is less than the thickness of the first non-woven layer 12, so that the toothed portion 38 penetrates into the first non-woven layer 12 through the second surface 20, while the first non-woven layer 12 advances at speed V1, retained by the outer surface 26 of the transport device 24. Preferably, the toothed portion 38 moves slower than the speed of the first non-woven layer 12 so that the toothed portion 38 engages the fibers of the first non-woven layer 12 and volumizes the first non-woven layer 12, that is, it produces an increase in the specific volume.

The extent of penetration of the toothed portion 38 into the thickness of the first non-woven layer 12 can be varied as a function of the difference in speed ΔV. In general, the extent of this penetration is preferably decreased with an increase in the difference in speed. In the extreme case in which the toothed portion 38 moves in the opposite direction to the direction of movement of the first non-woven layer 12, the difference in speed ΔV would be maximum, and the extent of penetration of would be conveniently decreased so as to obtain the degree of volumizing required.

The increase in the specific volume of the first non-woven layer 12 generated by the first volumizing device 34 and the suction action of the suction chamber 28 produce a deep penetration of the superabsorbent granular material 14 within the first non-woven layer 12 and a substantial increase in the density of superabsorbent granular material 14 that can be distributed in the first non-woven layer 12.

It can be noted that the suction chamber 28 has an extension that covers at least the portion of the outer surface 26 between the feed area of the first non-woven web 12 and the area in which the formation of the absorbent structure 10 is completed, thus including the transport device areas 24 facing the dispensing device 32 and the first volumizing device 34. The suction chamber 28 may be divided into sectors with different degrees of vacuum, in order to vary the suction action at different areas of the transport device, for example to have greater suction action at the dispensing device 32 and the first volumizing device 34.

In the case in which the superabsorbent granular material 14 is applied intermittently in the longitudinal direction, an actuation of the volumizing device 34 can be provided, in phase with the dispensing device 32. For example, the volumizing device 34 can be operated at variable speeds, with a peripheral speed lower than the feeding speed of the non-woven web 12 in the sections where the superabsorbent granular material is present 14, to increase the voluminosity of the non-woven web 12, and with a peripheral speed equal to the feeding speed of the non-woven web 12 along the sections where the superabsorbent granular material 14 is not present, so as not to alter the voluminosity of the non-woven web 12.

Alternatively, the volumizing device 34 may be movable orthogonally to the surface of the non-woven web 12, so as to be in contact with the non-woven web 12 in the sections where the superabsorbent granular material 14 is present, and detached from the non-woven web 12 along the sections where the superabsorbent granular material 14 is not present. This second solution is mechanically more complicated than the first, which is preferred.

Downstream of the first volumizing device 34, a feeder 40 is arranged, which feeds the second non-woven layer 16 onto the second surface 20 of the first non-woven layer 12, at a speed equal to the speed V1 of the first non-woven layer 12. In the example of FIG. 2, a glue dispenser 42 applies a small amount of glue on the surface of the second non-woven layer 16, which comes into contact with the second surface 20 of the first non-woven layer 12. A roller 44 is placed downstream of the feeder 40 and applies a pressure on the second non-woven layer 16 to complete the attachment. Alternatively, the second non-woven layer 16 may be fixed to the first non-woven layer 12 by means of thermomechanical or ultrasound welding. In this case, downstream of the feeder 40, the absorbent structure 10 is detached from the transport device 24 and is made to pass between two welding rollers. In this case, the glue dispenser 42 is not present. Alternatively, the second non-woven layer 16 may be fed between the outer surface 26 of the transport device 24 and the first non-woven layer 12, and fixed to the latter with one of the methods described previously.

The apparatus 22 may be provided with a second volumizing device 46 placed upstream of the dispensing device 32. The second volumizing device 46 comprises a wheel 48 with a toothed portion 50 that, as for the first volumizing device 34, is preferably movable in a concordant manner relative to the first non-woven layer 12, with a speed V3 less than the speed V1 of the first non-woven layer 12. The second volumizing device 46 operates in an identical manner to the first volumizing device 34 and produces an increase in the specific volume of the first non-woven layer 12 before dispensing the superabsorbent granular material 40. The increase in the specific volume of the first non-woven layer 12 upstream of the dispensing device 32 is advantageous for increasing the density of superabsorbent granular material that can be distributed inside the first non-woven layer 12. The speed V3 can be equal to the speed V2 of the first volumizing device 34 or it can be different, in order to have different volumizing effects upstream and downstream of the dispensing device 32. The second volumizing device 48 can be especially useful in the case where the first non-woven layer 12 is fed from reels 30 wound with a relatively high tension, which would be advantageous to increase the length of the material contained in the reels 30.

The inclusion of a second volumizing device 48 upstream of the dispenser 46 is a preferred but optional characteristic. The second volumizing device 48, if provided, may be positioned so as to act on the non-woven web 12 before it is supported by the outer surface 26 of the transport device 24.

In one embodiment, the apparatus 22 could be provided with only one volumizing device 46 placed upstream of the dispensing device 32. In this case, the volumizing device 34 placed downstream of the dispensing device may be omitted.

FIG. 3 shows a second embodiment of an apparatus 22 for producing the absorbent structure 10 of FIG. 1. The elements corresponding to those previously described are indicated with the same reference numerals.

In the case illustrated in FIG. 3, the transport device 24 comprises a belt having a flat outer surface 26 that carries the first non-woven layer 12 at a speed V1 along a rectilinear direction. As in the previously described embodiment, the apparatus of FIG. 3 comprises a first volumizing device 34 placed downstream of the dispensing device 32 and, optionally, a second volumizing device 46 placed upstream of the dispensing device 32. In this case as well, a suction chamber 28 is provided, facing the portion of the outer surface 26 that extends from the feed area of the first non-woven layer 12 and the area in which the absorbent structure is completed 10. In this case as well, a single volumizing device could be provided, placed upstream of the dispensing device 32.

In FIG. 4, numeral 10′ indicates a second embodiment of an absorbent structure without fluff. The absorbent structure 10′ comprises a first non-woven layer 12′, a first superabsorbent granular material 14′ dispersed between the fibers of the first non-woven layer 12′, a second non-woven layer 16, a third non-woven layer 12″ and a second superabsorbent granular material 14″ dispersed between the fibers of the third non-woven layer 12″. The second non-woven layer 16 is placed between two mutually facing surfaces of the first and the third non-woven layers 12′, 12″. The first non-woven layer 12′ and the third non-woven layer 12″ can have different densities and/or different types of the respective superabsorbent granular materials 14′, 14″.

FIG. 5 illustrates an apparatus 22′ for producing the absorbent structure 10′ of FIG. 4. The apparatus 22′ includes two transport devices 24′, 24″ comprising respective wheels with respective outer surfaces 26′, 26″, which carry the first and the third non-woven layers 12′, 12″ at a speed V1 in concordant directions. Each transport device, 24′, 24″ is associated with a respective dispensing device 32′, 32″ and with a respective volumizing device 34′, 34″. The volumizing devices 34′, 34″ have respective toothed portions 38′ and 38″, which in the illustrated embodiment are preferably movable in a direction concordant to the respective non-woven layers 12′ and 12″ with respective speeds V2′, V2″, which can be the same or different from each other, lower than the speed V1 of the first and third non-woven layers 12′, 12″. The transport devices 24′, 24″ are associated with respective suction chambers 28′, 28″.

The second non-woven layer 16 is fed at a speed V1 between the first and the third non-woven layers 12′, 12″. The apparatus 22′ is formed of two symmetrical sections with respect to the plane of the second non-woven layer 16. Each of the two sections functions in a similar way to that described previously.

The first, the second and the third non-woven layers 12′, 16, 12″ are fixed to each other by means of a welding device 52. Alternatively, the first, the second and the third non-woven layers 12′, 16, 12″ may be fixed together by gluing. In this case, glue dispensers would be provided for applying glue on the opposite surfaces of the second non-woven layer 16 and in place of the welding device 52, a pair of pressure rollers would be provided. Second volumizing devices can also be provided in the apparatus 22′ to increase the specific volume of the first and the third non-woven layers 12′, 12″ upstream of the respective dispensing devices 32′, 32″.

The absorbent structure 10′ can be advantageous in cases in which it is required to make absorbent cores with greater thickness or with differentiated absorbent capacity in the thickness thanks to a different density and/or type of the superabsorbent granular material 14′, 14″.

In the absorbent structure 10′, the second non-woven layer 16 may be omitted, so that the first and the third non-woven layers 12′, 12″ may have respective surfaces directly in contact with each other.

In general, each volumizing device incorporated into the apparatus of the present invention operates in a manner similar to that described with reference to the first volumizing device 34. In particular, the toothed portion is movable relative to the non-woven layer on which it acts so that there is a difference in speed ΔV between the speed of the non-woven layer and the speed of the toothed portion. Preferably, in the embodiments illustrated in FIGS. 2, 3 and 5, the toothed portion of the volumizing device is movable in a direction concordant to the direction of movement of the non-woven layer, at a lower speed than the speed of the non-woven layer.

FIG. 6 illustrates an apparatus 22 in which the first non-woven layer 12 is formed in line instead of being fed from a reel. In FIG. 6, the elements corresponding to those previously described are indicated with the same reference numerals.

In FIG. 6, numeral 54 indicates a forming device placed upstream of the transport device 24. The forming device comprises a conveyor belt 56 comprising a perforated belt 57 having an upper branch facing a stationary suction chamber 58. The forming device 54 comprises a distributor of fibers 60 that distributes measured quantities of fibers on the perforated belt 57. The fiber distributor 60 forms the first non-woven layer 12 on the surface of the perforated belt 57, which is fed to the transport device 24 of the apparatus 46 in a manner similar to that described previously.

The formation of the first non-woven layer 12 may be carried out on the periphery of a wheel instead of a conveyor belt.

The formation of the first non-woven layer 12 can take place directly on the surface of the perforated belt 57. In this case, an optional second non-woven layer 16 can be applied on a surface of the first non-woven layer 12 downstream of the first volumizing device 34, as previously described.

Alternatively, as shown in FIG. 6, an optional second non-woven layer 16 coming from a reel can be fed onto the surface of the perforated belt 57 upstream of the fiber distributor 60 and the fibers coming from the fiber distributor 60 are deposited on the surface of the second non-woven layer 16. The fibers forming the first non-woven layer 12 can be anchored to the second non-woven layer 16 by means of an adhesive layer applied to the surface of the second non-woven layer 16 upstream of the fiber-dispensing device 60.

The forming device 54 for the in-line formation of the first non-woven layer 12 can be applied to an apparatus 22 according to any of the embodiments illustrated in FIGS. 2, 3, 5. The characteristics of the two non-woven layers 12, 16 and the modes of volumization of the first non-woven layer 12 are the same as described previously.

Given that the first non-woven layer 12 is a highly voluminous material, in the case where the first non-woven layer 12 is fed from reels, there is the drawback of frequent replacements of the reels given that the high volume of the material implicates a limited length of the reels. The in-line formation of the first non-woven layer 12, as shown in FIG. 6, overcomes this drawback.

Of course, without prejudice to the principle of the invention, the details of construction and the embodiments can be varied with respect to those illustrated here, purely by way of non-limiting example, without departing from the scope of the invention as defined by the attached claims. 

The invention claimed is:
 1. A method for producing an absorbent structure for absorbent sanitary articles, comprising the steps of: advancing a first non-woven layer at a first speed, with a first surface of said first non-woven layer facing a suction chamber; distributing a superabsorbent granular material on a second surface of said first non-woven layer opposite to said first surface while said first surface is disposed facing said suction chamber; and increasing a volume of said first non-woven layer with a first toothed portion that penetrates into said first non-woven layer through said second surface, moves with respect to said first non-woven layer at a second speed different from said first speed, and is disposed downstream from distributing the superabsorbent granular material.
 2. A method according to claim 1, wherein said first toothed portion moves in a direction concordant with said first non-woven layer, said second speed being lower than said first speed.
 3. A method according to claim 1, further comprising a step of increasing said volume of said first non-woven layer upstream of said step of distributing superabsorbent granular material.
 4. A method according to claim 3, wherein the step of increasing said volume of said first non-woven layer upstream of said step of distributing superabsorbent granular material comprises increasing a volume of the first non-woven layer with a third toothed portion that penetrates into said first non-woven layer through the second surface of said first non-woven layer opposite to said first surface and that moves with respect to said first non-woven layer at a third speed different from said first speed.
 5. A method according to claim 1, further comprising the step of fixing a second non-woven layer to said second surface of said first non-woven layer.
 6. A method according to claim 5, wherein said second non-woven layer has a mass per unit area between 8 and 20 g/m² and a thickness between 0.1 and 0.4 mm.
 7. A method according to claim 1, further comprising the steps of: advancing a third non-woven layer at said first speed, with a first surface of said third non-woven layer facing a suction chamber; distributing superabsorbent granular material on a second surface of said third non-woven layer opposite to said first surface; increasing a volume of said third non-woven layer with a second toothed portion that penetrates into said third non-woven layer through said second surface and that moves with respect to said third non-woven layer at a third speed different from said first speed; and fixing together said first non-woven layer and said third non-woven layer.
 8. A method according to claim 7, wherein said toothed portion moves in a direction concordant with said third non-woven layer, said fourth speed being less than said first speed.
 9. A method according to claim 7, further comprising the step of placing a second non-woven layer between said first non-woven layer and said third non-woven layer.
 10. A method according to claim 1, wherein said superabsorbent granular material is distributed on said first non-woven layer in an intermittent manner in the longitudinal direction.
 11. A method according to claim 10, wherein the volume of said first non-woven layer is increased in an intermittent manner in phase with said step of distributing superabsorbent granular material.
 12. A method according to claim 1, wherein said first non- woven layer has an average specific volume between a minimum of 25 cm³/g and a maximum of 120 cm³/g.
 13. A method according to claim 1, wherein said first non-woven layer has a thickness between 2 and 12 mm.
 14. A method according to claim 1, further comprising the step of forming said first non-woven layer in line.
 15. A method according to claim 1, wherein the absorbent structure does not comprise cellulosic fluff. 